The partial oxidation of solid carbonaceous fuels such as coal and/or petroleum coke (petcoke) to produce mixtures of CO and H2 is a common practice. Within the gasifier, the carbonaceous feedstock is reacted with a controlled, substoichiometric quantity of oxygen in a—carbon rich environment. The specific operational processes vary depending on the type of gasifier employed and the desired CO and H2 composition.
In a slagging gasifier, nonvolatile impurities from the feedstock coalesce and form a viscous slag. The gasifier temperatures are typically optimized between about 1325 and 1575° C. to allow the slag to flow down the refractory lined walls, avoiding clogging and premature shutdown, and minimizing degradation of the refractory materials lining the gasification chamber. The carbonaceous fuel utilized as feed is typically coal, or a mixture of coal and petcoke, with the composition of the resulting slag closely related to the nonvolatile impurities present in the feedstock. Typical coal ashes generally contain significant amount of silicon, -aluminum, and iron, with substantially no Vanadium. In contrast, petcoke ashes generally contain lesser amounts of silicon, and a significantly increased amount of Vanadium. In typical ash analysis the quantities are reported as a weight percent (wt. %) of the respective oxide formed under oxidizing conditions, such as silica (SiO2), alumina (Al2O3), ferrous oxide (FeO), and vanadium pentoxide (V2O5), and under this nomenclature, coal ash compositions are generally comprised of about 45-50 wt. % SiO2 and substantially no V2O5, while petcoke ash compositions are generally comprised of a reduced SiO2 content and generally greater than about 20 wt. % V2O5. Additionally, coal contains approximately 10 wt. % nonvolatile impurities, whereas petcoke contains approximately 1 wt. % on average. As a result, in coal-petcoke mixtures, the overall slag quantity decreases as more petcoke is added, while the amount of petcoke slag in the dramatically increases.
The dramatic increase in petcoke slag as additional petcoke is utilized results in increased amounts of V2O5 entering the slagging gasifier as petcoke ash. This V2O5 is reduced to V2O3 under the reducing conditions of the gasifier, and correspondingly generates increased V2O3 levels in the resulting slag. Since V2O3 has a high melting point of about 1970° C., greater amounts of V2O3 in the slag will cause the melting temperature of the slag to increase. See e.g., Nakano et al., “Phase Equilibria in Synthetic Coal-Petcoke Slags (Al2O3—CaO—FeO—SiO2—V2O3) under Simulated Gasification Conditions,” Energy Fuels 25 (2011); and see Nakano et al., “Crystallization of Synthetic Coal-Petcoke Slag Mixtures Simulating Those Encountered in Entrained Bed Slagging Gasifiers,” Energy Fuels 23 (2009). The presence of high melting temperature V2O3 in the slag has a significant impact on the resulting slag viscosity of the slag at typical operating temperatures, which is typically treated as a key parameter for gasifier operations. As a result, feedstock composition is often optimized based on the gasifier temperatures necessary in order to maintain a relatively low viscosity slag, in order to maintain satisfactory slag drainage and avoid clogging, premature shutdown, and material degradation, and correspondingly, the petcoke content of carbon feedstocks is typically limited when no additional additives are used. Viscosity can be decreased to increase slag flow by raising the gasification temperature, but this has the negative effect of increasing refractory wear in the gasifier lining, causing increased system downtime.
Various additives have been employed in order to increase the liquidity of slag generated by feedstocks having no or limited Vanadium content. Calcia (CaO) and magnesia (MgO) have been investigated in reducing environments on an ash mixture comprised of greater than about 40 wt. % SiO2 and an absence of Vanadium, mixed with CaO contents between 5-20 wt. %. The CaO was found to decrease the melting temperature of the slag in highly reducing environments, with the magnitude of the decrease relatively constant irrespective of the wt. % of CaO added. See Wei et al., “Effect of Additives on Slag Properties in an Entrained Bed Gasifier,” presented at World of Coal Ash (WOCA) conference, May 9-12, 2011, Denver, Colo. Additionally, for petcoke feedstocks having less than about 10 wt. % CaO in the glass forming compounds, addition of CaO at the rate of 0.2-0.4 pounds per ton of petcoke feedstock has been recommended. See U.S. Pat. No. 5,578,094 to Brooker et al. Additions of MgO and manganese oxide have also been reported. See U.S. Pat. No. 8,197,566 to Meschter et al. In all cases demonstrations were limited to ash compositions having an absence of Vanadium. Generally, when CaO has been utilized as an additive for gasifier operations, quantities have been relatively limited, and the impact of CaO on the viscosity of silicate melts has been limited to silicate structure alteration from a three-dimensional network to discrete anionic groups. See e.g., Zhang et al., “Review and Modeling of Viscosity of Silicate Melts: Part I. Viscosity of Binary and Ternary Silicates Containing CaO, MgO, and MnO,” Metall. Mater. Trans. B 29B (1998), among others. Additionally, as is understood, CaO is extensively utilized in steelmaking for the neutralization of alumina, silica, sulfur, phosphorous, and other impurities typically found in metal ores, where Vanadium content is substantially absent.
Disclosed here is a method for the operation of a slagging gasifier using a carbon feedstock, where the carbon feedstock ash is relatively low in SiO2 and comparatively high in Vanadium content, such as the composition typically found in petcoke. The method limits the SiO2 content in the resulting slag in order to increase the V2O3 dissolution and limit SiO2 interactions with basic oxides such as CaO and FeO, and additionally utilizes a CaO additive to increase the solubility of V2O3 into slag. The increased V2O3 dissolution generated by the reduced SiO2 content in conjunction with the CaO additive acts to produce a slag of reduced viscosity and a reduced melting temperature for slags generated by high Vanadium content feedstocks, such as petcoke. The methodology thereby provides for the use of increased petcoke concentrations in carbon feedstocks utilized for the slagging gasifier, as well as allowing for slagging gasifier operations at reduced temperatures.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.